Study On The Elemental And Isotopic Geochemistry Of Sulfide In Fule Lead-Zinc Deposit, Northeast Yunnan

The Fule Pb-Zn deposit is located in the southeastern Sichuan-Yunnan-Guizhou Pb-Zn polymetallic metallogenic province.And the geotectonic position is on the north boundary of the Yangtze block and the South China fold belt.In this paper,detailed mineralogical observations were carried out based on systematic geological investigations using optical microscopy,scanning electron microscopy,and electron microprobe,then nine metal sulfides and two metal oxides were identified in this deposit,namely sphalerite,galena,pyrite,tetrahedrite,tennantite,chalcopyrite,vaesite,gersdorffite,Zn-bearing vaesite,cerussite and malachite.The sequence of minerals formation is divided according to the interlacing relation of minerals and the special mineral structure.The composition and distribution characteristics of major and trace elements and in situ sulfur isotope of the above-mentioned minerals were determined by SEM,EPMA,and LA-ICPMS.The formation environment of each mineral was analyzed in conjunction with previous research,and the source of the mineralized material and the genesis mechanism was explored,which providing rich mineral elemental and sulfur isotopic information for clarifying the genesis mechanism of the same type of Pb-Zn deposits in the region and the establishment of the mineralization model.The main findings of this study are as follows:（1）Important progress has been made in mineralogy:Sedimentary pyrite was discovered,and it is confirmed that pyrite was formed in two stages:sedimentary diagenetic stage（Py I）and hydrothermal stage（Py II）.Py I is a framboidal aggregate wrapped by Py II,forming the zonal structure.According to crystal morphology,Py II can be further divided into two-dimensional crystal（short columnar and strip）,cube,pentagonal dodecahedron,and anhedral.These pyrites with different morphologies show obvious morphological zonation characteristics.Tetrahedrite and tennantite are symbiotic with late galena（chalcopyrite）,mainly filling in the cracks of sphalerite or growing along the crystal edge of early sphalerite/galena.Vaesite,gersdorffite,and zinc-bearing vaesite are acicular and radially distributed in hydrothermal dolomite.Microscopic observation shows that the above-mentioned three nickel minerals are also produced in zonal structure,with vaesite in the core,the transition zone is gersdorffite,and the edge is zinc-bearing vaesite.（2）Geochemical characteristics of mineral elements:The sphalerite is relatively enriched In Cd,Ge,Ga,Cu,and Sb and poor in Fe,Pb,In,Co and Ni.Py I is enriched in Co,Ni,As,Mn,Tl,and other elements.The contents of Cu,Pb,Zn,Ni,As,Sb,Ge,and Se are higher in Py II.The tetrahedrite and tennantite have higher As and Zn,and the content of Zn is generally about 8%.Nickel sulfide minerals are generally enriched in As,and Cr is high in gersdorffite.LA-ICPMS images of trace elements show that Fe,Mn,and Cd(Fe²⁺+Mn²⁺（?）2Zn²⁺)replace Zn into the sphalerite lattice in the form of isomorphism.Ge（Ga）may replace Zn by n Cu²⁺+Ge⁴⁺(Ga³⁺)→（n+1）Zn²⁺,resulting in higher Ge and Ga content in sphalerite.In pyrite,Ge,Cd,and Zn exist in the form of microinclusion,and Se replacing S is enriched to a certain extent.（3）In situ sulfur isotope:theδ³⁴S value of sulfide in the deposit varies from-34.3 to 20.5‰.Theδ³⁴S of Py I ranges from-32.9‰to-34.3‰,and that of Py II ranges from+9.8‰to+20.5‰.Theδ³⁴S values of sphalerite range from+14.3 to+18.6‰.Theδ³⁴S values of galena range from-0.4 to+16.4‰.Theδ³⁴S value of tetrahedrite ranges from 0 to 8.5‰.Theδ³⁴S value of nickel sulfide varies from-6 to+12.3‰.The sulfur isotopic compositions of sulfide indicate that the reduced sulfur in the ore-forming fluid is derived from the reduction of Marine sulfate,and there may be multiple sulfur reservoirs.The formation mechanism of reduced sulfur in Py I is biological sulfate reduction（BSR）,while Py II,sphalerite,and galena are thermal sulfate reduction（TSR）.（4）Mineral formation environment:According to the characteristics of mineral elements,sulfur isotope,and occurrence location,it is considered that the different morphology of Py II in the deposit is the result of temperature,fO₂ and saturation of ore-forming fluid,of which temperature is dominant.As a result,the size and morphology of pyrite particles show a transition from the center to the edge of pentagonal dodecahedron→pentagonal dodecahedron+cube→cube→two-dimensional crystal,and then the particles gradually become smaller.The high content of As in fahlore and the presence of As in the form of anions indicate that the mineral was formed in a relatively oxidized environment.（5）Based on the geological characteristics,mineral assemblage,framework and structure of the ore,as well as the characteristics of mineral elements and sulfur isotope,it is considered that the Fule Pb-Zn deposit was formed in the compressional background,and is an MVT Pb-Zn deposit with stratabound,epiconozoic,rich disperse elements,and super high Pb+Zn grade of carbonate host ore.